[tab_capture] Change LOG(DFATAL) to LOG(WARNING) if fail to find new parents for offscreen tab.

chrome/browser/extensions/api/tab_capture/tab_capture_registry.cc

Index: chrome/browser/extensions/api/tab_capture/tab_capture_registry.cc
diff --git a/chrome/browser/extensions/api/tab_capture/tab_capture_registry.cc b/chrome/browser/extensions/api/tab_capture/tab_capture_registry.cc
index c415de6e71da2bf9da465f097dead6b1fa25b302..a42bf6c32ee95b705d59f7bf6d6b43999074cc1b 100644
--- a/chrome/browser/extensions/api/tab_capture/tab_capture_registry.cc
+++ b/chrome/browser/extensions/api/tab_capture/tab_capture_registry.cc
@@ -106,8 +106,8 @@ class WindowAdoptionAgent : protected aura::WindowObserver {
                << " adopts the content window " << content_window_ << '.';
       root_window->AddChild(content_window_);

[miu, 2017/02/24 22:51:55]

Some work has been done in content::DelegatedFrameHost, where capture now comes
directly from the Surface. This may mean we don't need this WindowAdoptionAgent
hack anymore.

So, on your desktop, can you comment out this LOC; and then run the
browser_tests to confirm whether offscreen tabs still render and capture?

  ninja -C out/dir browser_tests
  browser_tests --gtest_filter='*TabCaptureApiPixelTest*Offscreen*'

If they still work, let's simply delete the WindowAdoptionAgent class. :)

[zhaobin, 2017/02/25 00:30:44]

browser_tests passed if comment out this LOC. But when trying to start an
offscreen presentation, my cast device shows a green screen. It seems that
WindowAdoptionAgent is still needed.

     } else {
-      LOG(DFATAL) << "Unable to find an aura root window.  "
-                     "Compositing of the content may be halted!";
+      LOG(WARNING) << "Unable to find an aura root window.  "
+                      "Compositing of the content may be halted!";
     }
   }